Hydrobiologia 458: 75–82, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 75 The nutrient enrichment of Lake Victoria (Kenyan waters) H. Lung’ayia, L. Sitoki & M. Kenyanya Kenya Marine and Fisheries Research Institute, Kisumu Research Center, P.O. Box 1881, Kisumu, Kenya Key words: Eutrophication, degradation, limnology, conservation, sustainable management Abstract Lake Victoria is an international freshwater resource of great ecological and economic importance. Large changes that have occurred recently in the environment and the ecology of the lake are associated with natural processes as well as anthropogenic activities in the catchment’s area and the Lake itself. Data collected in the Kenyan waters of the Lake in June 1998 indicate that nutrient concentrations and algal biomass are higher than previously reported and that Secchi transparency has decreased. These are evidence of increasing nutrient enrichment and general decline in water quality. Increased algal blooms, depletions of dissolved oxygen, frequent large-scale mortalities of fish, decline of endemic fish species, predominance in the fishery by introduced species, changes in other biotic species composition, proliferation of the water hyacinth and increasing human health risks also manifest the ecological degradation of the Lake. Long-term conservation efforts for rehabilitation and sustainable management of the Lake are suggested. Introduction Lake Victoria is shared between Kenya, Uganda and Tanzania, which control 6%, 45% and 49% of the Lake respectively. The Lake lies between 3 ◦ S–0 ◦ 30 ′ N lat- itude and 31 ◦ 40 ′ E–34 ◦ 50 ′ E longitude at an altitude of 1134 m above sea level and has a surface area of 68 800 km 2 (mean depth 40 m and maximum depth 79 m). This makes it the second largest freshwater lake in the world and the largest in Africa. The catchment’s area of 195 000 km 2 , that also includes Rwanda and Burundi, has a high human population whose activities influence the lake intensively. Lake Victoria is a very important water resource in the region and supports the livelihood of millions of people in the riparian states. It is a major source of water supply for domestic and industrial purposes. The Lake supports a valuable fishery that supplies fish protein to East Africa as well as export markets in European Union (EU) countries, Israel, Australia and the U.S.A. (Muli, 1996). Trade in ornamental fish for aquaria is developing. The lake is also an avenue for transport and is increasingly becoming a tourist destin- ation due to sport-fishing, scenic beauty and wildlife of the area. Unique habitats harbour a rich variety of organisms and this contributes greatly to both the scientific and conservation interest of the Lake. A hydroelectric power plant that supplies electricity to Uganda and Kenya is located at the lake’s outlet to the White Nile. An increasingly variable climate in the Lake Vic- toria region, high population growth and develop- mental activities mainly agriculture, urbanization and industrialization contribute greatly to degradation of both land and water. This will continue to have dir- ect and indirect effects on the lake’s environment and ecology. Indeed, notable changes have occurred in the physical, chemical and biological regimes of the lake when compared with condition before and during the 1960s. The characteristics of the waters indicate nutrient enrichment. According to Hecky (1993), con- centration of nitrogen has become higher particularly in inshore waters. On the other hand, concentra- tion of silicon in the epilimnion of offshore regions has reduced by a factor of ten (Kilham & Kilham, 1990). The phytoplankton community that showed clear seasonal successions of diatoms, blue-green al- gae (cyanobacteria) and green algae and several other taxa (Talling, 1965, 1987) is now persistently predom- inated by cyanobacteria (Hecky, op. cit., Lung’ayia